why does the speed of transfer fluctuate that large

sometimes fall to 4-5MB/s, sometimes rising up to 100MB/s
, moving from SDD to HDD

Is it a sign of possible aging or failing of the HDD ?

 

What kind of files are you moving? Different file sizes will affect transfer speeds. Lots of small files usually results in slower speeds, where larger files usually result in faster speeds.

 

Looks like pure sequential operation over large file(s). (1 item remaining ,1.99GB)

Are both drives internally connected via SATA/PCIe or is the HDD in an external USB enclosure? If the later your USB port might be underpowered.

 

There so many things going on here one can't narrow it down. You could have hardware interface problems or cabling problem or Security software checking the transfer instead of minding it's own business or the file itself is so fragmented that the systems it bogged down search for the file. So unless someone is actually there to see that is going on nothing more can be had here.